This invention concerns asphalt paving mixing plants, and more specifically, to aggregate proportioning measurement and control means for an asphalt mixing plant having plural aggregate storage bins for different size aggregates. It also concerns systematic procedure for mix design, mix analysis, and job control for producing a uniform asphalt pavement of substantial length having uniform physical properties.
Asphalt surfaces generally are used to provide a riding surface for land vehicles or a take-off/landing surface for air vehicles. The primary material constituents of asphalt-hot-mix paving mixtures are mineral aggregates of different sizes and asphalt cement, an oil-base ingredient. The hot mix is prepared at a mixing plant which preferably is located near the site of the pavement to reduce transportation costs.
The aggregates generally are excavated from earth deposits, transported to a crushing plant for crushing and separated according to their size and finally stockpiled for subsequent use. Certain aggregates, such as river sand and hydraulically tumbles stone are not crushed but instead, are stockpiled for use in their present form immediately upon excavation. Evacavation and stockpiling often occur near the mixing plant, but in certain instances, the stockpiled aggregates are again transported to a remotely situated mixing plant and again stored in stockpiles near the mixing plant for use during mixing operations wherein the aggregates are supplied to separate storage bins of the mixing plant. During mixing, the aggregates are proportioned according to a predesignated job mix formula and a predesignated quantity of asphalt cement. The entire combination of aggregates and asphalt cement then is heated to a certain mixing temperature of about 250 degrees to 280 degrees Fahrenheit, and when thoroughly mixed and heated, the mixture is dumped from the bottom of a mixing bin into a dump truck that transports the hot mix to the pavement site. The hot mix then is picked up by laydown equipment and evenly spread on a prepared pavement subgrade or, in the case of an overlay, on top of a preexisting asphalt or concrete surface. Heavy steam rollers then compact the hot mix to produce the pavement. The compacted surface is allowed to cool for about one day prior to being put into service.
During handling of the aggregates as aforestated, a certain amount of segregation and degradation occurs. Efforts are made with difficulty to minimize segregation and degradation. Segregation is the separation of the larger aggregates from smaller aggregates within the blended mixture, while degradation is the degrading of larger sized aggregates into smaller sized aggregates.
Segregation naturally occurs when a blended mixture of aggregates is poured or dumped from the mixing plant into a truck bed, during transportation of the aggregates from the mixing plant to the road site, dumping of the hot mix aggregate blend at the road site, and also during handling by the laydown machine. Heavier aggregates have a natural tendency to be slung and ejected with greater force and therefore take a different path of travel during handling operations such as dumping, scooping and excavating. For example, when a hot mix is dumped from a truck into a pile, the heavier aggregates have a tendency to roll towards the sides and periphery of the pile whereas the finer aggregates tend to fall and collect at a location vertically downward.
Degradation, on the other hand, at worse, occurs within the aggregates prior to being introduced into the mixing bin of the mixing plant. For example, the larger aggregates of a stockpile have a tendency to strike one another and break up into smaller pieces during handling between the stockpile to the mixing plant. Accordingly, the proportioning of aggregates according to size is rather difficult. Degradation for coarse aggregates are known to exceed normally 30% between the stockpile and the output of the mixing plant. Finer aggregates, such as blended sand and crushed fines which pass the number 4 screen size do not degrade during handling.
During the production of asphalt paving mixtures, the paving engineer attempts to maintain the aggregate blend within certain predesignated specifications as previously indicated. A job mix formula is usually designated by the contracting authority as a series of percentages associated with a number of sieves which describe the aggregate blend. A typical job mix formula may be designated as 100% passing in the 3/4" sieve, 80-100% passing in the 1/2" sieve, 70-90% passing in the 3/8" sieve, 55-73% passing in the No. 4 screen, 40-55% passing the No. 8 screen, 20-30% passing the No. 30 screen, 10-18% passing the No. 100 screen, and 4-10% passing the No. 200 screen. Any blend of aggregates within the range designated by the job mix formula specifications generally is acceptable by the contracting authority under present standards. Difficulties are encountered in meeting job mix formula specifications because of segregation and degradation of aggregates as previously indicated, and further, due to lack of adequate feeding controls for the separate storage bins.
At the mixing plant, the separate storage bins of the mixing plant contain an aperture and feed belt of the bottom thereof which regulates the flow of aggregates into the mixing bin. Calibration normally is performed with dry aggregates. A certain rate of flow of dry aggregates is calibrated by adjusting the relative speeds of the individual feed belts associated with each storage bin. However, because stockpiles normally are stored out of doors and are not shielded from climatic conditions, a certain amount of moisture generally is present in the aggregate stockpiles. Because conventional aggregate feeding mechanisms are calibrated to dry aggregate flow rather than wet aggregate flow, certain errors in the blended composition may occur. Hopefully, the aggregates of the stockpile are prepared so that the actual blend will fall within the middle of each percentage specified in the job mix formula, but due to segregation and degradation as previously explained, adequate control of gradation almost is impossible with present systems. It is possible to adjust the feeding controls in accordance with the amount of moisture content in the aggregates, but this procedure is rather complicated in that the moisture content varies on a daily basis and also varies according to the climatic conditions. Calibration procedures to align all of the separate feed systems are complicated and time consuming. Furthermore, the amount of moisture retained in aggregates varies according to the size of the aggregate. Particularly, blend sand and crushed fines will retain more moisture than coarse and intermediate sized rock. Thus, in order to more accurately control the feeding operations from the feeding bins, feeding must be controlled accordingly to the size of aggregates. The prediction of aggregate size in the various stockpiles and the feeding bins is complicated by segregation and degradation problems which were previously set forth.
Using present calibration techniques for proportioning aggregates and by adjusting for moisture content, I have been able to maintain an accuracy such that 50% of my samples taken from the mixing bin were within 1% of tolerance, 90% of the samples taken were within 3% of tolerance, and 10% of the samples taken were within 5% of tolerance. All samples were measured over the no. 4 screen. Some engineers do not even take account of the moisture content of the aggregate stockpiles.
Improper control of aggregate proportioning at the mixing plant causes non-uniformity in the compacted mixture with respect to voidage control, flexibility, and stability. The lift of the pavement is extremely shortened, particularly at certain sections, thereby foreshortening the lift of the entire pavement. The pavement is deteriorated by aeration of the asphalt cement in the compacted mixture which results in oxidation of the asphalt cement. The compacted pavement also is subjected to water seepage which strips the coating of asphalt cement from the aggregate thereby causing the pavement to break up and lessen its flexibility.
In the plant-mix type pavement construction, a road bed and/or subgrade on which an asphaltic concrete mixture is laid generally are prepared by grading, compacting, and leveling. The asphaltic mixture is prepared at a remote plant site and is transported to the prepared road bed. The mixture is then dumped in place, spread by a paving machine, and finally compacted in place by heavy steam rollers.
Plant-mix production consumes large quantities of materials, typically as much as 6000 tons per day, thus possibly depleting any given single aggregate source, such as a rock quarry, sand pit, or other source of mineral deposits. Aggregate materials of pavements of several miles or more generally are gathered from several sources and accordingly nonuniform pavements result from source variations. Furthermore during the initiation of a production run, several miles of pavement typically are laid before the materials engineer has acquired sufficient knowledge of the aggregate qualities to establish consistent physical properties in the pavement. In many cases, consistent qualities may never be achieved either because the production job is completed before the material engineer has had the opportunity to correct for source variations, or the nature and quality of the aggregate changes too rapidly for the materials testing engineer to perform required tests and determine appropriate corrective action at the mixing plant. In many cases, the aggregate source intentionally is changed to shorten the distance between the aggregate source and the mixing plant whereupon attempts are then made to bring the new mixture within job specifications.
At the mixing plant, the amount of asphalt cement injected into the mixing bin can be reasonably accurately controlled as a percentage by weight of the total mix. Effective asphalt cement governs the amount of air voids in the compacted mixture and varies as a function of their shape, absorption characteristics, and sizes. Attempts are made to regulate size by controlling gradation over a multitude of screen sizes, generally 8 to 10, ranging from coarse aggregates passing the 1" screen down to mineral filler passing the no. 200 screen. Gradation however is almost impossible to control the job mix formula at the mixing plant and at the lay-down site due to degradation and segregation of aggregates. Plant control in accordance with changes in absorption characteristics, aggregate shapes, or other natural qualities are at best based upon trial-and-error techniques.
No method previously exists for quickly measuring changes in the natural aggregate qualities at the mixing plant site to effect corrective action at the mixing plant during operation. Further, no method previously exists for quickly and accurately determining the necessary corrective action for effecting such proper control of the production plant at the plant site. Thus uniform asphalt pavements cannot be produced.
Frictional qualities also are difficult to achieve for dense graded mixes. An open graded overlay is now used for friction overlays. Frictional qualities are provided by the sharp edges of 1/2" to 1" coarse aggregate particles protruding upwardly of the riding surface. The asphalt cement in the present open graded frictional overlay is permeable to water and air penetration causing the cement to age prematurely resulting in deterioration of the pavement structure. The underlying surface of the friction overlay also deteriorates for the same reasons. One problem associated with attempts to lay dense graded friction courses is that the coarse rock is pushed down into the underlying asphalt pavement upon load application on the riding surface and thus frictional qualities disappear. Thus to reduce the effects of displacement and reorientation of aggregate particles, the present friction overlay is laid very open or coarse.
The design of optimum asphaltic mixture today is at best a trial-and-error procedure. Good mixes generally result from knowledge of aggregates, experience, and luck. Many limitations in achieving optimum pavement properties exist, even with the expert design engineer using known aggregates. With some aggregates, present techniques cannot be used to determine a mix design that meets job specifications, such as stability, flexibility, density or voidage.
In an effort to produce uniform pavements, firstly mix design is performed by trial-and-error methods to determine what aggregate blendings asphalt cement combination will produce certain predesignated pavement qualitites such as flexibility, stability, and air voidage; and secondly, crushing, mixing, and laydown operations are controlled to produce an asphalt pavement having the desired physical properties. The object is to produce a pavement so that the load is transmitted to the subgrade only through the rock while the mortar of fine aggregate and cement fill the intersticies between the rock. The aggregates are proportioned according to a predesignated job mix formula which comprises prescribed proportions of different sized aggregates generally ranging from a maximum size of one and one-half inch in diameter to a minimum aggregate size that passes the no. 200 sieve.
Two methods now in general use for mix design and quality testing are the Marshall and the Heevm methods. According to these methods, test specimens are prepared by compacting trial mixes within the limitations of predesignated job mix formula tolerances, and then tested in an effort to determine which blend possesses desired physical properties, such as flexibility, flowability, density, stability, etc. This procedure consumes as much time as two to three weeks. It must be repeated when the natural qualitites of the aggregates change during the production operation or when different degrees of segregation and degradation of aggregates occur during stockpiling, transporting and handling.
An asphalt pavement containing a large amount of air voids is an "open graded mix". It is vulnerable to deterioration due to water seepage throughout the pavement. Water weakens the binding effect of the asphalt cement by stripping the cement from the surface of the aggregates. An open graded mix also is subject to aeration which shortens the pavement life due to oxidation of the asphalt cement. Aeration decreases the viscosity of the cement, causing it to become brittle and ultimate breakup of the pavement. Where prolonged road life is not important, an open graded mix, within certain limits, however is used to produce a high friction riding surface. Road pliability having resilient characteristics is sacrificed in open graded mixes.
The nature of the coarse aggregates determine, among other things, the load bearing and riding characteristics of the pavement. Specific qualities of concern include hardness, shape, porosity, and other surface qualities. Coarse aggregate may consist of crushed rock, volcanic rock, hydraulically tumbled stoned, or other large mineral deposits.
A pavement having a small amount of air voids is known as a "dense graded mix". This mixture has a larger quantity of the fine aggregate/cement mortar. Present dense graded mixtures have poor stability and deform under load forces as the coarse aggregates do not properly transmit loads to the subgrade. Where load conditions permit, pavements comprising dense graded mixes are used when high pliability is desired.
A maximum "bulking point" is defined herein as the proprtion (percentage by weight of dry coarse aggregate with respect to total weight of dry aggregate in mixture) at which each coarse aggregate particle touches one another while the mortar of fine aggregate and asphalt cement fill all interstitices between the coarse aggregate. The gradation at which the bulking point occurs essentially depends on the shape of the coarse aggregate. For example, the available volume of space between contiguous hydraulically tumbled stone is different than the available volume between contiguous crushed rock, and thus a mixture including one type of coarse aggregate may have a different bulking point than a mixture of another type of coarse aggregate. A mix at or above the bulking point mixture generally is unacceptable because it lacks, among other things, sufficient flexibility, and resistence to air and water penetration. Some presently specified job mix formulas completely ignore bulking point limitations in the specification.
A "balance point" is defined herein as the gradation at which each coarse aggregate particle are in close proximity to one another while the mortar of fine aggregate, asphalt cement, and desired dispersed air voids fill all interstitices between the coarse aggregate. There should be just enough asphalt cement in the mixture, and just enough air voids to allow for proper expansion and contraction of the pavement under the climatic conditions without penetration of excessive water and air. Thus the air voids are the separating factor between dense and open graded mixes below the bulking point. Excessive asphalt cement reduces stability and too little asphalt cement generally reduces flexibility. The voidage control is difficult to achieve and maintain during the mixing operation because of myriad variables, some of which have been previously indicated, and accordingly, uniform pavements cannot be produced by present techniques.
At least one procedure presently in use for designing a mixture which has the desired quantity of air voids is the manipulation of the job mix formula of discrete sized aggregates over the entire gradation scale. Specifically, Fuller Maximum Density Curves and the Federal Highway Administration 0.45 Power Gradation Chart currently are in use. Their use is described in "Mix Design Methods for Asphaltic Concrete and other Hot-Mix Type" published by The Asphalt Institute, manual series no. 2, fourth edition, March 1974. The basis for the FHA 0.45 power gradation chart is described in detail in volume 31, pages 176 through 207 of the "Proceedings of the Association of Asphalt Paving Technologist", Jan. 29, 1962. The theory of controlling voidage is based on the principle that a gradation deviating from a maximum density curve will contain increased air voids. Thus some obscure relationship between job mix formula and voidage is developed for job control. It is rarely successful particularly in view of segregation and degradation of aggregates while being handled and transported to and from the crushing plant, stock piles, mixing plant, and laydown site. Degradation between the crushing plant and the mixing plant alone may amount to as much as 30%.
Another method for computing voids in the mineral aggregate is disclosed in volume 34, pages 574 through 594 of the "Proceedings of the Association of Asphalt Paving Technologist". The computations are based upon successive correlations of voids in discrete ranges of the gradation spectrum. Page 577 of the treatise illustrates 8 gradation ranges between mineral filler and 3/8 to 3/4 inch aggregate. By summing the voidage contained in each aggregate group, and considering the correlation factors of aggregate voidage, a sum total is obtained which closely approximates the final aggregate voidage. No clue however is given to how one might achieve this ideal combination of aggregate mixture at the mixing plant nor is any consideration given to variation in effective asphalt cement.
In any job, the use of local indigenous materials is desired, if at all possible, because of expensive transportation cost in transporting aggregates across the country. Certain jobs however do not lend themselves to the use of such indigenous materials because of widely varying changes in natural aggregate qualities that render mixing plant control almost impossible. In that case, materials are imported from a distant source.